In a wide variety of industrial, commercial and military environments, the ordinary concrete, asphalt, wood or other working surfaces are inadequate to withstand the punishing weather and the day-to-day use to which they are subjected to. Loading docks and sidewalks in shipyards, railroad yards, factories and the like are constantly battered, chipped and cracked by the impact of heavy objects such as grappling hooks, heavy clevices, chains, metal containers and frequently by the lugs of steel wheeled machinery. Highway surfaces under the influence of weather and heavy use become cracked and pitted, sections are popped out and joints opened up creating increased vulnerability to further damage.
An example of particularly hard use and severe impact to a working surface is the deck surface on an aircraft carrier in the area immediately under the aircraft arrestor cables and grappling hooks as well as similar landing strips at airports which are utilized in the landing of planes. Such surfaces are ordinarily paved with specially-formulated reinforced concrete but they must nevertheless be replaced all too frequently at great costs and loss of operating time for the carrier and similarily used land based air strips.
Of the numerous alternatives that have been considered and tested for this and similar applications, one broad group or family of materials that has offered more promise than most encompasses a variety of epoxy-based materials.
Epoxy resins have many valuable properties which suit them admirably for such applications. They adhere well to nearly any surface, cure easily, exhibit low shrinkage, are chemically inert, heat and moisture resistant, hard and have an unusual degree of flexibility and impact resistance.
All resins of this type contain the epoxy atomic group in which an atom of oxygen is linked by two of the atoms in a carbon chain as, for example, in the case of ethylene oxide. They are formed in a process of polymerization or epoxidation that begins most typically with bisphenol A and epichlorohydrin as raw materials. These substances polymerize to form low-molecular weight chains. The action of a curing agent, usually a polyamine or a polyamide, links and extends the chains. Because no volatile product is split out during these reactions, there is minimal shrinkage. Other monomers containing the epoxy group and other curing agents also can be used. Special catalysts and other additives are employed including polysulphides, polyamines, amino and phenolic resins.
The McGraw-Hill Encyclopedia of Science and Technology (1971) describes epoxidation as a process by which olefinic material may be converted to opoxy (oxirane) compounds by a variety of methods. These methods include direct oxidation in the presence of silver catalysts, dehydrochlorination of chlorohydrins and reaction with peracids. The last method is most commonly employed. Depending on the peracid used and reaction conditions, the reaction may proceed beyond the formation of epoxy compounds with the production of hydroxyacyloxy compounds or glycols.
Great strides have been made in recent years in the development of improved epoxy materials, but for the applications of interest here, even the best formulations fall short of performance goals. While shrinkage is indeed minimal it is typically excessive for application as a surface over large areas because of the cracking and separation that inevitably results. An additional limitation is the universal tendency of currently available epoxy materials to lose their initial flexibility as aging progresses. This is believed to result from the use of catalysts such as polysulphides whose activity continues throughout the useful life of the material. A parallel example is the eventual loss of strength in concrete as it ages excessively, eventually crumbling and falling apart.
It thus becomes apparent with support from experience that further improvements in epoxy materials are essential to their successful application in the difficult service herein addressed.